Interganglionic cerebral-buccal mechanoafferents of Aplysia: receptive fields and synaptic connections to different classes of neurons involved in feeding behavior

Motor control of the appetitive phase of feeding behavior in Aplysia

The appetitive phase of feeding behavior, in the gastropod, Aplysia, consists of head lifting, head waving, orientation of the head to food, and locomotion. We have initiated studies of the neural control of head waving using three methods: (i) anatomical description of the nerves innervating muscles that are involved in head movement, (ii) electrical stimulation of nerves in a semi-intact preparation, and (iii) recording from nerves in free-moving animals. The muscles controlling head movements, located in the dorsal and lateral neck region, are innervated primarily by pleural nerve 1 and pedal nerves 2, 3, and 5. Electrical stimulation of these nerves caused both longitudinal and lateral contractions of the neck muscles, the largest contractions being in the area where the nerve first enters the muscle. Extracellular recordings from pleural nerve 1 and pedal nerves, in free-moving animals, showed an increase in extracellular activity during head lifting, at the onset of appetitive feeding behavior. Directionally specific inhibition and excitation in neural activity occurred in pleural nerve 1 and pedal nerve 5 during leftward and rightward movements of the head (head waving). Cobalt and nickel backfills of pleural nerve 1 and pedal nerve 5 revealed cell bodies in the cerebral, pedal, and pleural ganglia. The neurons are therefore putative motor neurons for the neck muscles involved in appetitive behavior. This evidence suggests that appetitive control of feeding may involve the coordinated activity of several different ganglia.

Cellular and synaptic morphology of a feeding motor circuit in Aplysia californica

The cellular and synaptic morphology of a component of the feeding motor circuit in Aplysia californica was examined with light and electron microscopic techniques. The circuit consists of a pair of inhibitory premotor interneurons, B4 and B5, as well as two motoneurons, B15 and B16, which innervate the accessory radula closer muscle. The neurons have wide, varicose arborizations in the buccal ganglion neuropil. All four of these neurons are cholinergic, and in addition, B15 contains immunoreactivity to sera raised against small cardioactive peptide B. Varicose processes in the accessory radula closer muscle are immunoreactive with antisera against several neuropeptides. We identified specific neuromuscular junctions by visualizing horseradish peroxidase uptake in recycled synaptic vesicles. Direct innervation of the accessory radula closer muscle by B15 and B16 is demonstrated by experiments in which horseradish peroxidase is transported from motoneuronal soma to the terminals on muscle fibers. In addition, specific synaptic contacts between B4 and B5 and each of the motoneurons are observed in the buccal ganglion neuropil. Finally, multiple contacts consistent with peptidergic, serotoninergic, and cholinergic synapses are made onto the neurons, suggesting that a variety of transmitters modulate motor output at each level of the hierarchical circuit. These results support the physiological evidence suggesting the involvement of neuropeptides as well as "classical" transmitters in the modulation of circuitry governing feeding behavior in Aplysia.

Input-output relationships of identified buccal neurones involved in feeding control in Aplysia

A group of about 28 neurones located in the lateral portion of the caudal face of Aplysia buccal ganglion and projecting into the cerebro-buccal connective were identified by retrograde cobalt staining, and designated as L neurones. It was found that the L neurones did not establish synaptic relations with the known buccal neurones, which are mainly involved in the production of the consummatory phase of feeding, nor with several cerebral neurones tested, including the well-known serotonin giant cell. Neither did they show responses to stimulation of the nerves directed to the buccal mass. On the other hand, the L neurones showed depolarizing responses, with the possible addition of a weak, slower hyperpolarizing phase, to stimulation of the ipsi- and contralateral oesophageal nerves, which innervate the portion of the gut posterior to the buccal mass. These findings, together with several properties of the oesophageal nerve input, suggest that one function of the L cells is to transmit information about gut regions posterior to the buccal mass towards the cerebral ganglia, and that they may mediate the inhibitory influence which in Aplysia is known to be exerted upon feeding by the presence of bulk in the anterior gut. The L neurones showed synaptic responses - consisting mainly or exclusively of depolarizations - to stimulation of the cerebro-buccal connectives. Besides this, large, tonic EPSPs, which often occurred in the 'spontaneous' activity of the L neurones, were found to be generated by spikes that travelled in the cerebro-buccal connective towards the buccal ganglion.(ABSTRACT TRUNCATED AT 250 WORDS)